We will characterize the clinical dose-response curves for the relief of parkinsonism and the production of dyskinesia as a function of levodopa and 3-O-methyldopa levels in ten patients with advanced Parkinson's disease before and after undergoing stereotactic posteroventral pallidotomy. We will test the hypothesis that pallidotomy selectively shifts to the right the levodoa dose-response curve for dyskinesia without altering the dose response curve for the relief of parkinsonism. Recruitment is ongoing.